Microwave Spectrum, Internal Rotation, Dipole Moment, Quadrupole Coupling, and Structure of Nitrosomethane

The microwave spectra of CH₃NO and CD₃NO have been observed and analyzed. The barriers to internal rotation are 1137 ± 5 and 1095 ± 7 cal/mole for CH₃NO and CD₃NO, respectively, a difference of 42 cal/mole which is thought to be real. The barriers were calculated by the internal‐axis method with retention of higher terms in the usual Fourier expansion of the rotational energy and computation of the torsional integrals in the harmonic‐oscillator approximation. The theoretical parameters were fitted to the data by a least‐squares method and the uncertainties reported are standard deviations. Transitions having comparable Stark and quadrupole energies were used to calculate the dipole moment. Secular equations of second or third order were solved for each of the data points. The dipole moment components are $μa = 2.240 ± 0.001 D, μb = 0.522 ± 0.006 D$⁠, and $μtotal = 2.300 ± 0.002 D$⁠. The quadrupole coupling constants of CH₃NO are 0.50 ± 0.16, −6.016 ± 0.031, and 5.518 ± 0.031 for the components along the $a, b$⁠, and $c$ axes, respectively. The corresponding values for CD₃NO are 0.60 ± 0.07, −6.007 ± 0.020, and 5.411 ± 0.020. The N=O and C–N distances are not well determined by the data. The CNO angle is 112.6 ± 1.0°.